Amazon's Kindle DX is a perfect device for one such purpose, and believe that the demand for this product would grow because there are many people who likes to read on the move. The DX has 4GB internal memory which can accomodate up to 3,500 ebooks on average. It is thin and easy to slip into the bag without causing any discomfort while carrying.

Features
# Slim: Just over 1/3 of an inch, as thin as most magazines
# Carry Your Library: Holds up to 3,500 books, periodicals, and documents
# Beautiful Large Display: 9.7" diagonal e-ink screen reads like real paper; boasts 16 shades of gray for clear text and sharp images
# Auto-Rotating Screen: Display auto-rotates from portrait to landscape as you turn the device so you can view full-width maps, graphs, tables, and Web pages
# Built-In PDF Reader: Native PDF support allows you to carry and read all of your personal and professional documents on the go
# Wireless: 3G wireless lets you download books right from your Kindle DX, anytime, anywhere; no monthly fees, no annual contracts, and no hunting for Wi-Fi hotspots
# Books In Under 60 Seconds: You get free wireless delivery of books in less than 60 seconds; no PC required
# Long Battery Life: Read for days without recharging
# With the text-to-speech feature, Kindle DX can read newspapers, magazines, blogs, and books out loud to you, unless the book's rights holder made the feature unavailable

How about using the DX in the classroom as a replacement for textbooks? We did not think that it is a great idea because a notebook should be more appropriate, but it is not impossible. The DX should be used where mobility matters in a tight constraint space, such as in the train.